Generation of power using emissive materials



June 30, 1964 c M. HENDERSON ETAL 3,139,541

GENERATION OF POWER USING EMISSIVE MATERIALS Original Filed July 5, 19603 Sheets-Sheet l r: 1 I g 5 i,- E CDC [2 O 20 I Z & O o r- 0 X J) 2| v)1| 2 n: I 22 O l- 9 TEMPERATURE- TEMPERATURE FIGURE l FIGURE 2 Z T a 30K- 5 5 s g 3,. I) Z 5 s 3 32 1 2\ N I 9 26 cc '5 27 8 N 0 Lu TEMPERATUREFIGURE 3 FIGURE 4 INVENTORS COURTLAN D M. HENDERSON ROBERT G. AULT BVW QW ATTORNEY June 30, 1964 c. M. HENDERSON ETAL 3,139,541

GENERATION OF POWER USING EMISSIVE MATERIALS Original Filed July 5, 19603 Sheets-Sheet 2 FIGURE 5 FIGURE 6 2 70 U 73 7| a: LL]

a 7 /4 74 CL 2 TEMPERATURE FIGURE 7 INvENToRs HGURE I2 COURTLAND M.HENDERSON ROBERT G. AULT BY Maw FIGURE II ATTORNEY June 30, 1964 c. M.HENDERSON ETAL 3,139,541

GENERATION OF POWER USING EMISSIVE MATERIALS Original Filed July 5, 19603 Sheets-Sheet 3 AMPS PER CM NUMBER OF |ooc:ooo|o0c THERMAL CYCLESFIGURE 8 FIGURE 9 H3&

INVENTORS H COURTLAND M. HENDERSON ROBERT c. AULT FIGURE 10 v ATTORNEYUnited States Patent ()1 fice 3,139,541 Patented June 30, 1964 1 Claim.(Cl. 310-4) The present invention relates to emissive materials,processes for the production of same, and specific devices in which thesaid unique emissive compositions are an essential part. It is an objectof the invention toprovide electrodes and other emissive elements whichare characterized by high thermal and electrical conductivities, byunusually stable and high electron emissivities over a wide range oftemperatures, and which at the same time overcome problems of prior artmaterials such as sagging and corrosion of elements operated at hightemperatures, and of minimizing spalling or blistering of coatings usedto increase the emissivity of prior art materials.

The present invention is directed to emissive bodies such as electrodesused in thermionic devices in which the said electrodes are comprised ofa metal matrix consisting of at least one member selected from the classconsisting of nickel, iron, cobalt, tungsten, molybdenum, columbium,tantalum, chromium, vanadium, copper, silver, gold, platinum, andiridium having internally dispersed therein a refractory additive as areinforcing agent such as a metallic oxide, carbide, boride, silicide ornitride, particularly of the rare earth metals of the lanthanum group,thorium, titanium, zirconium, columbium, tantalum, hafnium, vanadium,molybdenum, and tungsten.

In a preferred embodiment of the invention the internally dispersedmodifying agent is a member of the class consisting of cerium oxide,neodymium oxide, praseodymium oxide, lanthanum oxide, thorium oxide, andmixtures thereof.

In a more preferred embodiment of the invention,

the intimately dispersed additives in the said metal matrix are basedupon very fine particles or nuclei of the ad ditives, e.g., oxides, suchas from to 500,000 Angstrom particle size. More preferredparticle sizeranges are 10 to 10,000 Angstrom microns, or if narrower fractions aredesired, 50 to 10,000 Angstrom, with the most preferred range being 50to 225 Angstrom particle sizes.

The concentration of the reinforcing components existing as a distinctphase as nuclei internally dispersed in the matrix metal is from 0.25%to 50% by volume, a preferred range being from 0.25 to 35% by volume.

The use of the present additives, e.g., the above oxides, has been foundto result in the production of especially efficient emissive bodies whenthe reinforcing component such as the oxides existing as nuclei in themetal matrix have an inter-nuclei spacing of 10 Angstroms to 200,000Angstroms, or preferably 10 Angstroms to 5,000 Angstrorns. I

Still more preferred ranges in the region of close nuclei spacing is theuse of an inter-nuclei spacing of 10 to 225 Angstroms.

It has long been recognized in the electronic industry, and particularlyin the manufacture of radio tubes that the useful life of such tubes isoften shortened by: (A) failure of the prior art metals and alloys tohave suflicient creep strength to prevent sagging and Warping of thecomponents that are required to operate at elevated temperatures; (B)the emissivities of prior art materials, whether operated as hot or coldcathodes, decreased with this severe service.

cathode materials, the'difference in coefiicients of expansion betweenthe metal and oxide coating is so great that spalling or blistering orflaking off of the coatings from the base is a common occurrence thatseriously shortens the life of emitters so constructed.

Similarly in the construction and operation of thermionic heat-to-energyconversion devices, it has been difficult to find cathode and anodematerials with the necessary combination of resistance to flaking, highthermal and electrical conductivities, high creep strength and highemissivities to permit the design of units for In the case of the vacuumtype thermionic energy converter, it is quite important that very closeand uniform spacings be maintained between the hot or emitting cathodeand the collector or cool anode. In this application operatingtemperatures ranging well in excess of 1,000 C. are desired in order toobtain maximum energy conversion efficiencies. Oxidation resistance, inaddition to high creep strength at high temperatures is quite desirablebut these properties are not attainable with prior art metals and alloysat temperatures much above 900 C. and in particular above 1,000 C.

To maximize eificiency of heat input to electrical output withthermionic units, it is important that the inner emitting surface of thehot cathode be maintained at the highest temperature feasible for agiven material. The heat resistant, high-temperature, high-strengthmetallic materials of the prior art are usually alloyed for strength andimproved corrosion resistance purposes. Such prior art materials ormetals exhibit notably lower thermal and electrical conductivities thanthe metals of this invention. In general the use of ceramic type hotcathodes is notfavored due to the lower thermal and electricalconductivities of such materials as comparaed with the improved metalsof this invention.

In the use of electron discharge devices, one of the common limitationsof the useful life is a decrease in the electron emissive capabilitiesof the cathode that is occasioned by the flaking or peeling off of thesurface coating or emissive oxide material from the surface of thecathode base metal. This problem is especially accentuated in dischargedevices employing nickel cathodes in a mercury vapor atmosphere, such asin Thyratrons, since amalgamation or wetting of the nickel by of heaterpower to emitter current. well known to the industry is that when eitheralloys or mercury tends to destroy the bond at the interfaces betweenthe nickel metal and the emissive oxide layers thereon. It thereforefollows that reduction of this flaking or peeling results in an improvedcathode imparting longer useful life to electron discharge devices.

Directly heated, thorium-activated thermionic emitters of filament typecomprised of tungsten Wire containing thorium oxide are well known inthe art. heated filamentary emitters have certain serious limitationsfrom both an electrical and mechanical standpoint. For example it isdiflicult to produce reliable structures characterized by goodmechanical strength, ease of assembly, uniform emission and freedom fromgasiness using prior art tungsten base metals. .Also many such emittershave been found to have an undesirably high ratio A further problemrelatively pure metals are coated by alkali earth oxides or thoriumoxide, such metals tend to develop interfacial layers with the alkalineearth oxides. Such an effect lowers the emissivities of this type ofprior art material.

Earlier workers in the field of cathode manufacture have incorporatedvarious bodies into the cathode, such as fibers or with a refractorymetal extending through an emitting oxide. Another modification was thesintering of Such directly 3 emitting oxide particles on a metal base,or conversely, sintered oxides used as a core which was then coveredwith a sintered metal layer. However, all of these cathodes have beenvery costly to fabricate and have been characterized by short life,nonuniform emissivity and mechanical failure.

The advantages and characteristics of the materials and processes ofthis invention with regard to overcoming the foregoing problems willappear in the ensuing description.

The present invention overcomes the foregoing disadvantages of the priorart electrodes by the provision of improved articles of manufacturewhich exhibit high emissivities, high electrical and thermalconductivities, and which retain said high emissivities as well as highcreep strength and resistance to distortion even at elevatedtemperatures. Furthermore, the present improved electrodes have theability to bond emissive oxide coatings, if a coating is optionallydesired, more tenaciously than prior art metals, and are more readilyprocessed and formed into intricate shapes than was generally with priorart metals and alloys.

The electrodes of the present invention are high strength articles ofmanufacture consisting of a matrix of at least one metal of the matricesdescribed above and having intimately dispersed therein a refractoryadditive. A preferred group of the said additives is an oxide selectedfrom the group consisting of cerium oxide, neodymium oxide, praseodymiumoxide, lanthanum oxide, thorium oxide, and mixtures thereof.

FIGURES l to 4, 7 and 8 depict characteristic curves of variouselectrode materials of the prior art and of the present invention.

FIGURES 5 and 6 are intended to contrast prior art electrodecoatingswith those of the present invention.

FIGURES 9 and 10 show the invention applied respectively to a thermionicand a thermoelectric power converter.

FIGURES 11 and 12 schematically represent the invention applied to gasdischarge devices.

Examples of oxide mixtures commercially available and of utility in theinvention have the following approximate compositions:

60% (by weight) of Nd O 17% Pr O 10% Sm O and 13% of other rare earthoxides consisting primarily of Gd O and CeO 41% (by weight) Pr -O 24% SmO 14% Gd O 5% Nd Ce0 and 11% of other rare earth oxides consistingprimarily of Y O and La O 50% (by weight) Ce oxide, 24% La oxide, 17% Ndoxide, and 9% of other rare earth oxides consisting primarily of Proxide, Sm oxide, and Gd oxide.

La203, Nd203, FY6011, 6% Sm O and 7% of other rare earth oxidesconsisting primarily of Gd O CeO Y O 95% (by weight) of ThO and 5% ofoxides consisting primarily of rare earth elements.

In general, the various commercially available mixtures of rare earthcompounds and the refractory additives derived therefrom may be used,with the above critical group of matrices to produce an improvedelectrode.

In the practice of the present invention, the specific dispersingrefractory material, such as the oxide, is the essential additive,although minor proportions of metals other than the matrix metals asdescribed above may also be present. The dispersed refractorymaterial'is employed either as a pure material or in various commercialmixtures wherein the said refractory material is the majo component.

The electrodes of the present invention are prepared by consolidating anintimate dispersion of the aforesaid matrix metal and the refractorymaterial. This may be based upon a mechanically blended mixture of thebase metal and the dispersed refractory material, or a mixture resultingfrom chemical precipitation, or coating techniques whereby either themetal or the refractory material is the core and the outer covering ofthe indicated particles. However, a preferred embodiment of theinvention is based upon the preliminary production of a mixture ofoxides of the matrix group and the oxide group by oxidizing a solutionof compounds of the respective components by volatilization andoxidation in a flame.

Such crude oxide mixture is then subjected to reducing conditions suchas by contacting with hydrogen gas to reduce the matrix metal whileleaving the oxide compo nent dispersed at a molecular level in themetal. The powder is consolidated by hot or cold pressing, extruding,rolling, impact or explosive forming, etc., to obtain the ultimateelectrode and supporting hardware. Thus, the

electrodes may have a unitary construction in which both the emissiveand the support elements are made of the above-described reinforcedmetal, or the present emissive electrode materials may be held in placeby posts or hardware such as nickel, tungsten, and other conventionalmaterials.

The following examples illustrate specific embodiments of the presentinvention and show various comparisons against prior art compositions,materials, and processes.

Example 1 One preferred method for preliminarily forming the startingmaterials of the present invention is to oxidize an atomized solution ofat least one soluble salt of the matrix metal selected from theaforesaid group with a salt selected from the said oxide components,e.g., of cerium, neodymium, praseodymiurn, lanthanum, thorium, andmixtures thereof. In the present specific example a salt ofpraeseodymium is dissolved in a solvent such as water or alcohol, thesaid oxidation being conducted by means of an oxidizing flame to produceparticles com.- posed of members selected from the group consisting ofthe free metals and oxides of the first group and the Pr oxide inmolecular combination and thereafter subjecting the said particles toreducing conditions, e.g., with hydrogen, to produce the said elementalmetal of the aforesaid group, having dispersed therein unreduced Proxide. For example, when nickel nitrate and Pr nitrate are dissolved inwater in the desired proportions, e.g., to yield 92% (by volume) nickelmetal and 8% Pr oxide in the final product, and the said solutions areatomized and oxidized in an oxidizing flame, a powder is produced whichis comprised of nickel oxide and Pr oxide. The Pr oxide is dispersedwithin the individual mixed oxide particles at a molecular level. Theforegoing combination of Pr oxide and nickel oxide is reduced at atemperature of from about 500 C. to 700 C. in a hydrogen containingatmosphere, preferably more than 8 volume percent hydrogen. Otherreducing atmospheres such as carbon monoxide, water gas, forming gas,etc., are also useful for this purpose. The nickel oxide issubstantially entirely reduced to metallic nickel with the Pr oxideremaining unaffected,

and being dispersed at the substantially molecular level within themicrostructure of the nickel as a matrix.

Example 2 As another example, when chloroplatinic acid and Ce nitrate inthe proportions desired in the ultimate product, e.g., 95% Pt and 5% Ceoxide are dissolved in water and the resulting solution atomized andoxidized in an oxidizmg flame, a powder is produced which is comprisedof platinum and Ce oxide. The Ce oxide is dispersed within the platinummatrix of the individual particles at a molecular level; This materialis readily fabricated to a shaped body under the pressure andtemperature conditions set forth herein, e.g., at about 1500 p.s.i. and1500 C. by hot pressing. After forming the powder comprised of the freemetal and having the additive oxide dispersed therein, a preliminaryfabrication or compacting step may be em.- ployed. This, for example,can consist of hydrostatic coinpaction, cold-pressing, or slip casting,as well as other con- .ing of from 10 to 200,000 Angstroms. embodimentof the invention, the reinforcing oxide is solidation procedures to forma densified green billet. Such billets are then consolidated further bysintering in the aforesaid reducing atmoshpere at temperatures of aboutthree-quarters of the melting point (absolute) of the metal matrixmaterial. It has been found preferable to use a reducing atmosphere (asby pure hydrogen or hydrogen diluted with nitrogen as obtained fromcracked ammonia) in this sintering operation.

The reduced free metal matrix with the molecularly dispersed oxide isconsolidated into a shaped body. 'Prcferred conditions for suchconsolidation are pressing at pressures ranging from 1,000 p.s.i. to500,000 p.s.i., the most preferred range of consolidation pressuresbeing 40,- 0:00 p.s.i. to 140,000 p.s.i. Temperatures for suchconsolidation may range from room temperatures to 95% of the absolutemelting point of the matrix metals. The application of pressure and heatmay be carried out simultaneously, as in hot-pressing or they may becompleted in individual consecutive steps. Other consolidation steps mayalso be carried out in order to density the product or to shape thepreliminary bodies into ultimate commercial shapes, such as wire, rods,sheet stock, and other structural shapes and fabricated devices, such aselectrical cores. Conventional fabrication techniques such ashotextrusion, hot-rolling to sheet stock, wire drawing to small diameterwire sizes, forging swaging, andother metal fabrication processes may bereadily employed to the appreciably better workability of the materialsof this invention as compared with common tungsten alloys, and othersimilar materials described in the prior art.

Example 3 The individual nuclei of the oxide in the broadest aspect ofthe invention are present with a nucleus-nucleus spac- In a preferredpresent in the consolidated metal in a molecular degree of dispersion asshown by X-ray diffraction data, with more than 80% of such oxide nucleiseparated at distances of from to 5,000 Angstroms. More preferrednucleus-to-nucleus spacing is of the order of from 10 to 225 Angstromsand such spacings are quite common in the present electrodes. Thesefigures have also been found to be applicable to the other metalmatrices and reinforcing oxides described above.

Example With regard to the improved creep strength of the electrodes ofthis invention when used as high temperature hot cathodes, FIGURE 1illustrates, via stress-rupture diagrams, the relative resistance tocreep or high temperature strength of several prior art alloys versus ametal typical of this invention. Curve 10 of FIGURE 1 depicts therelative low strength of a pure metal, for example, a typical nickelcathode material. Curve 12 shows that the strength of a typical metal ofthis invention for example, nickel strengthened with 8% by volume of(1e0 is greatly increased, e.g., nearly twice as strong and creepresistant as the commercial nickel commonly used as electrode posts,electrodes and grids.

Curve 11 shows a typical nickel alloy also used in emissive type devicessuch as radio tubes. The strength of this alloy is considerably less,over a broad range of temperatures, than that of the comparable metal ofthis invention shown in curve 12. Thermionic diodes made to operate at acathode temperature of 12001250 C., with a spacing of 0.0008 inchbetween large area type cathode and anode remained properly spaced anddelivered high power output for more than double the life of anidentical diode constructed with conventional alloy nickel coated withbarium oxide.

Example 5 The thermal and electrical conductivities of the metals ofthis invention, as compared with those for typical electrode metals andalloys are shown in FIGURES 2 6 and 3 in which values of thermalconductivity and electrical conductivity vs. temperature, respectively,are plotted. As shown in FIGURE 2, we have found, for example, that theuse of neodymium oxide when added at a concentration of 6% by volume toa matrix metal of molybdenum decreases the thermal conductivitybyapproximately 6%. This is shown by comparing curve 21, showing thethermal conductivity vs. temperature for a molybdenum-neodymium oxidemetal of this invention with curve 20 which shows the variation of thethermal conductivity of pure molybdenum vs. temperature. Curve 22 ofFIGURE 2 shows the low thermal conductivity of a conventional solidsolution type molybdenum-silver alloy as compared with pure molybdenumand a metal of this invention.

In FIGURE 3, curve 26, it is shown that the specific use of 6% by volumeof neodymium oxide in a metal matrix (e.g., molybdenum) gives electricalconductivities which are very close to that for pure molybdenum, curve25, and substantially greater than that obtained with an alloy ofmolybdenum and silver, curve 27.

Thus, by the addition of the dispersed oxide, it has been shown that itis possible to retain thermal and electrical conductivities nearly ashigh as for the low strength pure cathode metals while retaining, andeven exceeding, the high creep strengths of the poorly conduc ing butstrong cathode type alloys.

Higher thermal and electrical conductivities are also obtained when thepresent metal matrices, strengthened with the present refractorymaterials are compared with the same metals strengthened by conventionalalloying techniques.

Example 6 FIGURE 4 shows the relative resistance to oxidation by 6.5% byvolume cerium oxide in nickel, curve 30, as compared with a typicalnickel-chrome alloy, curve 31, under cyclic heating and quenchingconditions in which the specimens were heated rapidly in air to 1100 C.,held for one hour at 1100" C. in air, then air quenched to roomtemperature. The superiority of such cerium oxide-strengthened nickelover a conventional heat resistant alloy, curve 31, under such severeconditions is quite significant and of value for applications wheremetalsare to be used under even slightly corrosive or oxidizingconditions as might be occasionally encountered in improperly evacuatedtubes at temperatures up to and exceeding those used in this test. Forexample, as shown in curve of FIGURE 4, nickel strengthened with 6.5% byvolume of cerium oxide showed a leveling off tendency in percentgain-in-weight after 34 cycles. The strength of the ceriumoxide-strengthened test electrodes were apparently unaltered even after20 such thermal cycles in air. The conventional nickel-chromium alloy,as shown in curve 31, failed catastrophically after i Example 7 Anotherembodiment of the invention is the coated type of cathode, since thereare many applications in which an alkali earth oxide emissive surface isdesirable.

In such types of cathodes, the coating of the alkali earth oxide such asbaria or calcia is applied to a consolidated matrix of the present basemetals, e.g., columbium, which base metal has internally dispersedtherein 7% by volume of neodymium oxide. In the present example, theoxides exist as nuclei at a molecular degree of dispersion and have anucleus-to-nucleus distance of from to 2000 Angstroms, with 50% of thenucleus-to-nucleus spacing less than 500 Angstroms apart. The oxidenuclei ranged in size from 50 to 2000 Angstroms with in excess of 50% ofthe nuclei being less than 500 Angstroms.

The superiority of the metals of this invention with regard toovercoming the problem of minimizing spalling and flaking off ofemissive coatings from prior art base 7 metals used in electrodes wasfound to be dually effective.

For example, FIGURE shows the kind of flaking or spalling away of anemissive coating, 41 and 43, from a prior art cathode metal, 40, afteras few as 20 severe thermal cycles. Such loss of emissive coatingsseriously lowers the average emissivity of the cathode and lessens theefiiciency of devices utilizing such materials. The distance 42 betweenanchor or bonding points, 44, between the film and the base metal is toolarge to prevent flexing or blistering of the film away from the basemetal. Accordignly, the emissive film, 43, soon spalls oil, and its losslowers the efliciency of the device in which such materials are used. Bycomparison, in FIGURE 6 based on electrodes of the present invention thepresence of a very large number of molecularly spaced refractoryparticles, 53, in a matrix metal, 50, offers a very short space, 52,between anchor points for the emissive film 51. This close spacingbetween anchor points bonding the film to the base metal of thisinvention minimizes the degree of film flexing during thermal cyclingand thereby substantially eliminates thermal spalling and flaking.

A further point of improvement of the materials of this invention withrespect to obtaining better bonding of emissive coatings as comparedwith prior art materials is that of Wetting or bonding between emissivecoatings applied to the materials of this invention is more readilyachieved, especially when an emissive coating contains as a majorcomponent the refractory material internally dispersed as astrengthening agent in the matrix metal. The presence of the same(thoria) or mutually soluble components (strontia and ceria, e.g.) inthe emissive coating material and the electrode metal promote bondingwhich substantially eliminates coating losses.

Secondly, the use of the said'refractory compounds of this inventiondispersed so uniformly throughout the entire electrode or grid provideselectrodes and grids with emissivities that are superior to conventionalalloys and metals as well as electrodes and grids with highly emissivecoatings on conventional metals and alloys.

Example 8 As shown in FIGURE 7, the emissions obtained with a typicalmetal of this invention consisting in this case of molecularly sizedcerium oxide particles uniformly dispersed through the interior and onthe surface of nickel as a matrix metal are compared with the same ceriastrengthened nickel coated with barium oxide and several prior artmaterials. The ordinate of FIGURE 7 represents the saturation emissiondensities of the several types of emitters in amperes per squarecentimeter of surface, plotted to a logarithmic scale, while theabscissa shows the temperature of the emitter in degrees centigrade. Thecurves of FIGURE 7 were obtained by applying a DC. voltage of about 500volts with an anode spacing of 1 cm. and by measuring the cathodecurrent produced at various temperatures. Curve '70 represents theemission from an electrode of nickel plus ceria (10% by Volume) withnuclei sizes ranging from 70 to 3000 Angstrom and internuclei spacingranging from 50 to 1000 Angstrom. Curve 72 shows the variation ofemissivity of thoriated tungsten with temperature. Curve '71 representsthe emission from a cathode of lanthanum hexaboride. Curve '73represents the emissivity of a barium oxide coated cathode ofnickel-ceria like that shown in curve 70. Curve '74 represents the curvefor a tungsten cathode and curve '75 shows the emissivities of a bariumoxide coated commercially available nickel cathode. Specifically withinthe relations of FIGURE 7 a current density of 0.03 amps per squarecentimeter as obtained at 850 C. with the nickel-ceria, curve 70, isobtained only at substantially higher temperatures with prior arttungsten (2100 C.), thoriated tungsten (1450 C.) and lanthanumhexaboride (1200 C.). The dependence of emissivities on cathodetemperature between coated and fabrication.

uncoated nickel-ceria cathodes is not so great, showing that the ceriastrengthened nickel is good enough to be used without a coating, and issuperior to typical prior art cathode materials.

Example 9 The change in emissivity of typical prior art electrodematerials with thermal cycling is compared with the emissivity of twotypical electrode materials of this invention. This is shown in FIGURE 8where the change in cathode current density expressed as amps per squarecentimeter is plotted against the number of thermal cycles. Each thermalcycle consisted of quickly heating the cathode to 1100" C., holding thecathode at constant temperature for about 3 minutes, then quicklycooling the cathode to about 100 C. For example, under the conditions ofthe relations shown in FIGURE 8, curve shows that the cathode currentdensity of the nickelceria of curve '70 of FIGURE 7 remained practicallyconstant over better than 70 thermal cycles. The cathode current densityof the barium oxide coated nickel-ceria of this invention, curve 81,began to drop olT gradually after 45 thermal cycles, suggesting thatbonding of the barium oxide to the nickel-ceria remained tight and thatof the barium oxide content was possibly being lost only at a slow ratedue to evaporation. Curve 32 shows that a prior art barium oxide coatednickel alloy cathode lost most of its barium oxide coating after only 20thermal cycles. The comparative performance results shown in FIGURES 7and 8 clearly demonstrate the important improvements of the emissivitiesand life of the materials of this invention over those of the prior art.

Example 10 Another advantage of this invention over prior art methodsand materials is to be found in the manufacture of highly emittingelectrodes, particularly where prior art coating techniques areemployed.

The present invention is a great improvement over the coated cathodes ofthe prior art. Thus, the use of a painted or deposited coating ofoxides, carbonates, etc., of various salts on a metal base as employedin the past has been characterized by considerable difiiculties inmaintaining a uniform coating which maintains constant emissivitycharacteristics with continued use. It has been impractical to formspecial electrode shapes after the brittle refractory coatings areapplied. This limits the applications of prior art highly emissivecoated materials to shapes that may be coated only in final form.Theelectrodes of this invention, due to the fact that they contain thesaid refractory materials dispersed internally, uniformly'and closelythroughout the body of the metal electrode and because such materialsmay be readily hot and cold worked by conventional means into intricateshapes without damage to the emitting surface, ofier significantadvantages over prior materials and methods of For example, an electrodeof 0.1" diameter was readily produced from a tungsten matrix metal with4% by volume of ceria of particle size or nuclei ranging from 50 to 2500Angstroms with nucleus-to-nucleus spacings of 20 to 2000 Angstromswithin the body of the shaped electrode. This electrode exhibited a workfunction of 1.8 e.v. over a temperature ranging from 1200- 1500 C.Similar data for a conventional tungsten wire electrode yielded a workfunction of 4.5 e.v. over temperatures from 1200-1600 C. This shows thatunder similar conditions electrodes based on the present materials havea superior work function and a correspondingly higher emissivity.

Example 11 Another embodiment of the invention is the use of electrodesof the present invention for thermionic power genmatrix metal with thespecific internally dispersed oxides makes it possible to emit a flow ofelectrons across an evacuated or gas filled space to another electrodewhich in turn is connected through a circuit with the emittingelectrode, thus providing for a continuous supply of electricity. FIGURE9 shows a simplified schematic diagram of a thermionic generation systemin which 101 is a heat source which directs heat to the electrodeelement 102,

which upon being heated emits electrons 103 across the I vacuum in space104 to the relatively cooler electrode 105, made of the same internallyfortified material, or another similar material, or even a conventionalmetal, such as nickel or iron. Leads 106 and 107 attached to the twoelectrical elements, provide for contacts to an external circuit 108 toemploy the electric power which is produced.

The heat input to the thermionic power generating device may be fromvarious conventional sources, preferably at high temperatures such asnuclear or solar energy, as well as heat obtained from the combustion ofmineral fuels. In all such applications, corrosion resistance, andstability of the electrode emission material is a serious problem.Consequently the alloys used in this part for this service have been farfrom satisfactory. For example (nickel with volume of ceria) electrodeof the present invention based upon a metal matrix containing internallydispersed therein an oxide selected from the group consisting of ceriumoxide, neodymium oxide, praseodymium oxide, lanthanum oxide, thoriumoxide, and mixtures thereof, are particularly useful in this respect andovercome the prior art ditliculties. In each instance, the internallyreinforced metallic electrode is superior in refractory properties, thatis thermal stability, creep strength and resistance to corrosionrelative to the corresponding pure metal.

In the above example of the 5% ceria in nickel, the oxide nuclei rangedfrom 80 to 3000 Angstroms and the nucleus-to-nucleus spacing was from 70to 2000 Angstroms.

An advantage of the present electrodes for thermionic generation is thehigh electron emitting capacity concomitant with a low evaporation rate,thus providing for high electrical efliciency and long life in thissevere type of service.

In addition the high thermal conductivities and electricalconductivities, the excellent oxidation resistance, the highemissivities, the high strength at high temperatures, and the readyformability characteristics of the electrodes of this invention alsomake them applicable for use in thermo-electric energy converters asshown in FIGURE 10. Here, electrode 110, consisting of platinumstrengthened with 5% by volume of neodymium oxide, absorbs and transmitshigh thermal energy from a source, element 113 to element 111 consistingof an efficient thermoelectric material, gallium arsenide. Element 112removes the unused thermal energy from element 111 and due to its highemissivity rejects the waste heat to the atmosphere. Element 112 iscomprised of a copper matrix in which 4% by volume of A1 0 is dispersed.Leads 114 and 115 connect the electrodes 110 and 112 to an external load116, completing the circuit.

Cathodes of the materials described above constitute an importantembodiment of the present invention. Thus,

in the application of this improved electron emissive mabe present asdesired for various control purposes. For example, a nickel electrodewith 5% by volume of ceria is a representation of the present inventionbased upon a metal matrix containing internally dispersed therein anoxide selected from the group consisting of cerium oxide, neodymiumoxide, praseodymium oxide, lanthanum oxide, thorium oxide, and mixturesthereof, are particularly useful in this respect and overcome the priorart difiiculties. In each instance, the internally reinforced metallicelectrode is superior in refractory properties, that is thermalstability, creep strength and resistance to corrosion relative to thecorresponding pure metal.

In the above example of the 5% ceria in nickel, the oxide nuclei rangedfrom to 3000 Angstroms and the nucleus-to-nucleus spacing was from 70 to2000 Augstroms.

-An advantage of the present electrodes for thermionic generation is thehigh electron emitting capacity concomitant with a low evaporation rate,thus providing for high electrical eificiency and long life in thissevere type of service.

' Example 12 The drawings, FIGURE 11 and FIGURE 12, of the presentpatent application show a specific embodiment of the invention as anelectron tube or gas discharge device. I

The tube envelope, 120, of glass, ceramic, or metal contains electrodes121 and 122. The electrodes are shown as conventional structures,although various shapes are possible, and numerous electrodes may alsobe present in the envelope. However, the essential electron emittingelectrode, typically the cathode, 121, is formed from thedispersion-strengthened metal matrices described above. The anodeelectrode, 122, is formed either from a similar material or from aconventional metal. The anode is not subjected to the severe operatingconditions of the cathode as described above. FIGURE 12 shows avariation of the invention in which a coated cathode is used. This iscomposed of a base, 125, of the above dispersionstrengthened metalhaving a coating, 126, of another emissive material such as barium oxidedeposited thereon.

The tube also contains a gas filling which is ionized in the operationof the tube. However, the present electrodes are equally applicable tohigh vacuum tubes.

The present patent application contains subject matter which is alsoreferred to in copending applications Serial Nos. 27,542, 27,543,27,544, 27,545, and 27,546, filed May 9, 1960 and Serial No. 40,683,filed July 5, 1960. The present patent application is a division of thelattermost application.

What is claimed:

Process for generating electric power which comprises applying heat toone face of a body of a metal selected from the group consisting ofnickel, iron, cobalt, tungsten, molybdenum, columbium, tantalum,chromium, vanadium, copper, silver, gold, platinum and iridium, andhaving molecularly dispersed therein from 0.25% to 50% by volume of anoxide selected from the group consisting of cerium oxide, neodymiumoxide, praseodymium oxide, lanthanum oxide, thorium oxide, as a secondcomponent existing as nuclei of 10 to 500,000 Angstrom particle size,and which are a discrete phase with the nucleusto-nucleus distancebetween the oxide nuclei being from 10 to 225 Angstroms, the other saidface being in a space under vacuum conditions, and the said space alsohaving another electrode therein, at a colder temperature, andwithdrawing an electric current between the two said electrodes.

Hunter Aug. 15, 1933 Levi Jan. 18, 1955

